Film, sheets and containers produced from a vinylidene chloride copolymer composition are excellent in oxygen impermeability, moisture resistance, transparency, chemical resistance, oil resistance, adhesiveness and heat shrinkage, and further is excellent from the viewpoint of food hygiene, so that they are in use for a vast variety of food packaging materials and wrapping films.
However, since vinylidene chloride-based polymers are poor in heat stability, it was indispensable in the previous production of these films, sheets and containers to add several percents of a liquid plasticizer to the polymer in order to prevent thermal decomposition caused by frictional heat at the time of processing. The addition of a liquid plasticizer, however, causes deterioration of the barrier property, which is the most eminent feature of vinylidene chloride-based polymers, and at the same time inevitably increases the amount of plasticizer extracted by oily foods. Though the thermal decomposition due to frictional heat at the time of processing can be prevented even when scarcely any liquid plasticizer is added if the molecular weight of the polymer is extremely lowered, the resulting film has a very low mechanical strength and cannot be used in practice.
Further, for obtaining a vinylidene chloride copolymer composition which can be processed with addition of no, or a very small amount of, liquid plasticizer, there are known a process which comprises blending a vinylidene chloride-based polymer with a polymer other than vinylidene chloride-based polymers which has a low softening temperature and low melt viscosity, as disclosed in Japanese Patent Kokai (Laid-open) No. 53-16753, and a process which comprises internal plasticization of a vinylidene chloride-based polymer by copolymerizing an acrylic ester with the vinylidene chloride-based polymer to lower the glass transition temperature, as disclosed in Japanese Patent Kokai (Laid-open) No. 04-72314. However, monolayer films obtained by these processes are all unsatisfactory from the viewpoint of preventing the deterioration of barrier property.
Further, as disclosed in Japanese Patent Kokai (Laid-open) No. 55-156,058, a multi-layer film has been proposed which comprises a barrier property-imparting layer obtained from a vinylidene chloride copolymer composition having a low molecular weight and containing only a small amount of liquid plasticizer, and a mechanical strength imparting layer obtained from a vinylidene chloride copolymer composition having a high molecular weight and containing a large amount of liquid plasticizer. In this film, however, the barrier property-imparting layer has a low mechanical strength, and the cost of processing equipment is high, so that the film is not economically advantageous.